Synthesis and characterization of highly branched sulfonated poly(aryl ether ketone) copolymers as proton exchange membranes

Highly branched sulfonated poly(aryl ether ketone) copolymers (Br-SPAEKs) were synthesized based on 4,4-(hexafluoroisopropylidene) diphenol (A(2) monomer), 4,4-difluorobenzophenone (unsulfonated B-2 monomer), 3,3-disodiumsulfonyl-4,4-difluorobenzophenone (sulfonated B-2 monomer), and 3,4,5-trifluoro-benzophenone (branched BB2 monomer). Structures of Br-SPAEKs were confirmed by Fourier transform infrared spectroscopy and nuclear magnetic resonance. Compared to the linear SPAEK (L-SPAEK) copolymer membrane with similar ion exchange capacity, the Br-SPAEK-30 (branched monomer of 20mol% and sulfonated monomer of 30mol%) membrane showed higher proton conductivity and lower methanol permeability. This synergistic effect on proton conductivity and methanol permeability was mainly due to the characteristics of large free volume and similar cross-linking architecture arising due to the introduction of the branched monomers. Moreover, the selectivity of the Br-SPAEK-30 membrane is almost three times higher than that of the L-SPAEK membrane. Proton conductivities of Br-SPAEK membranes are higher than 10(-2)Scm(-1) and increase gradually with increasing temperature. At 80 degrees C, the proton conductivity of the Br-SPAEK-50 (branched monomer of 20mol% and sulfonated monomer of 50mol%) membrane reaches 0.127Scm(-1), comparable to that of Nafion-117 (0.139Scm(-1)). Additionally, the methanol permeability of Br-SPAEK-50 is one order magnitude lower than that of Nafion-117.